Magnetic Exchange between Orbitally Degenerate Metal Ions: The Problem of Magnetic Anisotropy

In this paper we show that a strong magnetic anisotropy appears in exchange mixed–valence clusters containing orbitally degenerate metal ions. Combining an effective Hamiltonian approach with the technique of the irreducible tensor operators (ITO) and pseudoangular momentum representation we have solved the problem of magnetic exchange in localized and delocalized (mixed–valence) systems with different overall symmetries (D2h, D3h, D4h). The energy pattern as well as the character of the magnetic anisotropy is closely related to the ground term of the ions, electron transfer pathways, and overall symmetry of the system being affected also by the local crystal fields, spin–orbital interactions, and vibronic interactions. Special attention is paid to the origin of the magnetic anisotropy in the face–shared (D3h) binuclear unit [Ti2Cl9]-3. For this case a very good agreement between calculated and measured CHI(T) is obtained.